1. Field of the Invention
The invention relates in general to a light emitting diode (LED) drive circuit, and more particularly to an LED drive circuit whose outputted drive voltage has a wider voltage range in the LED load by using the voltage regulating technology.
2. Description of the Related Art
Light emitting diode (LED) relates to a solid state light emitting element, which is formed by a semiconductor material and has the advantages of small size, low heat generation, high illumination, low power consumption, long lifespan and being applicable to mass production. Currently, most illuminating devices or backlight modules have used LED as light emitting source. As the application field of LED gets wider and wider, how to increase light emitting efficiency or reduce production cost to reduce the price has become a prominent task for the industries.
The switching power supply is the main design of drive circuit in the AC-to-DC power supply of the conventional LED illuminating device. The switching power supply uses a semiconductor unit as a switching element and controls the on/off duty cycle of the semiconductor unit by using the pulse width modulation (PWM) technology, such that the output of power supply can be stabilized.
According to different needs in practical application, the switching power supply is mainly divided into two types, namely, the isolated type drive circuit and the non-isolated type drive circuit. In the isolated circuit, there is no physical circuit connection between an input end of power supply and an output end of LED element, and a transformer outputs a voltage to provide necessary power for operation by way of electromagnetic induction. That is, the above two ends, which are the input and output ends, are electrically isolated from each other. In the non-isolated circuit, the two ends are connected by a physical circuit and are not electrically isolated from each other. Currently, the isolated circuit has various types such as flyback type, forward type, push-pull type, half bridge type, and full bridge type.
Ordinary single-stage flyback drive circuits with small to medium power, advantageously having simple structure, low manufacturing cost, isolated design conformed to safety standards, and compensation function in power factor correction (PFC), are conformed to the International Energy Efficiency Standards. The said drive circuit controls the stability in power output by using the PWM technology. However, the output voltage at the output end of the drive circuit used in the LED element is restricted by the range of working voltage of the semiconductor unit (that is, PWM controller or control integrated circuit), and cannot be used in the LED element requiring a larger range of output voltage.
To put it in greater details, under current technologies, the transformer can define a primary winding and a secondary winding at an input end of a power supply and an output end of an LED element respectively. The transform of DC voltage at the two ends is designed according to a fixed weight ratio, such as 1:2. When the voltage at one end is 10V, the voltage outputted to the other end is 20V. The PWM controller is located on the primary side of the transformer. Therefore, when the LED element needs a higher output voltage (for example, when the number of LED elements in series is increased), the voltage of the PWM controller will increase accordingly.
Given that the PWM controller is subjected to the restriction of a maximum voltage rating, when the voltage outputted to the PWM controller is higher than the maximum voltage rating, the PWM controller will stop controlling the output of the PWM signal, hence resulting in control errors or abnormal operations. For the LED element to receive a corresponding and appropriate output voltage, the transformer of the drive circuit needs to collaborate with different situations of practical application. For instance, the transformer with corresponding weight ratio is adopted in response to different numbers of LED elements in series. Under such circumstances, manufacturing and management will become more inconvenient, and production cost will increase accordingly.